Neutral beam injection has proven to be an effective way to heat plasmas in tokamaks as well as mirror devices. Multi-amperes of neutral atoms have already been obtained from deuterium ions for energies as high as 120 keV. In some future fusion reactors, such as the Mirror Fusion Test Facility at the Lawrence Livermore Laboratory, high currents of lower energy (40 keV) deuterium atoms are required in certain neutral beam lines. In that respect, it is more advantageous to form the neutral atoms from the molecular D.sub.2.sup.+ ions and accelerate them to twice the energy (80 keV). In passing through the gas neutralizer, these D.sub.2.sup.+ ions will first dissociate and will then be neutralized to form two atomic particles with half the original D.sub.2.sup.+ ion energy However, this technique is useful only if ion sources that can generate a high percentage (&gt;70%) of D.sub.2.sup.+ ions are available.